1. Technical Field of the Invention
The present invention pertains in general to trimming circuits for voltage controlled oscillators and, more particularly, to a trimming circuit operable at high frequencies and capable of integration with a voltage controlled oscillator on a single semiconductor integrated circuit chip.
2. Description of the Related Art
Trimming a center frequency of a voltage controlled oscillator is typically performed through the use of an external trimming circuit. The trimming circuit is located external to the voltage controlled oscillator on a discrete circuit to simplify adjustment of a resonance frequency. Today, however, there are increased demands to reduce the size and cost of radio equipment particularly in the radio telephone industry. To reduce the size and cost of the radio equipment, more and more functionality is being incorporated onto a single integrated circuit chip. For this reason it is desirable to integrate the trimming circuit onto a single integrated circuit chip together with the voltage controlled oscillator.
Trimming circuits currently can be placed on a single semiconductor chip with the voltage controlled oscillator through the use of a variable capacitor. A digital to analog converter is connected to a variable capacitor and a desired capacitance value associated with a desired center frequency is programmed into the digital to analog converter. The analog output of the digital to analog converter adjusts the variable capacitor in response to the programmed capacitance value thereby varying the resonance frequency of the trimming circuit. In turn, the new resonance frequency adjusts the center frequency of the voltage controlled oscillator.
Problems arise, however, as the supply voltage is decreased or when a wide trimming frequency range is desired. At low power supply voltages increasingly used in radio telephone equipment, for example two and seven tenths volts, it becomes very difficult to obtain the required trimming frequency range with currently available "on-chip" variable capacitors. Furthermore, as the desired trimming frequency range increases, noise created by the digital to analog converter used to adjust the variable capacitor becomes problematic. The inputs to the trimming circuit and the associated voltage controlled oscillator become increasingly sensitive as the trimming frequency range is increased requiring the level of noise created by the digital to analog converter to be kept extremely low. Therefore, as the trimming frequency range increases, it becomes increasingly difficult and expensive to design and manufacture the digital to analog circuit.
Another approach to varying the capacitance of the trimming circuit is through the use of diode switches. While diode switches have been used to connect and disconnect different capacitances to the trimming circuit, the diodes are constructed of discrete PIN diodes separate from the remaining oscillator functionality. The PIN diodes are not integrated onto a semiconductor chip with the oscillator since PIN diodes require a special process to fabricate which is not conducive to bulk integration.
It would be advantageous, therefore, to devise a trimming circuit operable at high frequencies and capable of integration with a voltage controlled oscillator on a single semiconductor chip. It would be further advantageous if such a circuit included a diode switch capable of operation at frequencies above two gigahertz and further capable of integration with the voltage controlled oscillator on a bipolar metal oxide semiconductor.